This invention relates to a private branch exchange that supports a packet exchange service of an ISDN (integrated services digital network).
A line switching service and a packet switching service have been proposed as data communication exchange services in a public network. A DDX-C (line switching service) is available as one of the line switching services, and a DDX-P (a packet switching service of a first kind), a DDX-TP (a packet switching service of a second kind) and an INS-P (information network system-P) (ISDN packet switching service) are available as packet switching services. A packet switching service has the following features in comparison with a line switching service:
(1) Terminals having different speeds can communicate with each other. Since the network has a store-and-forward switching function, data communication can be performed even when terminals have different communication speeds. PA1 (2) Terminals having different procedures can communicate with each other. By virtue of the fact that the network makes a conversion among communication procedures, even terminals having different procedures can communicate with each other. PA1 (3) Multiplexed communication of packets is possible. By changing addressee information, it is possible for a plurality of parties to communicate with one another using a single physical line. PA1 (4) Alternative routes can be selected. A route within a network can be selected by the addressee information even if a trunk circuit or the like fails during communication. PA1 (5) Users can be billed for the amount of information used. In other words, users are billed in accordance with information quantity (packet data).
Owing to the features mentioned above, packet switching services are being utilized, regardless of whether terminals employ different speeds and procedures, in conversational-type data communication such as communication between personal computers, various reservation systems, etc. Further, with the aforesaid INS-P, a packet switching service in available not only in a B channel, which is an information channel, but also in a D channel, which is a signal channel. The usefulness of this system is being advocated strongly.
For these reasons, users of packet switching systems have been increasing year by year not only in industry but in general.
In a case where a packet switching service is utilized by employing an ISDN basic access interface (of two B and D channels), it is possible to utilize both the line switching and packet switching services within a single interface (two B channels+one D channel) by entering into an agreement.
However, even with an agreement that allows use of both the line switching and packet switching services within a single interface, either one service is allowed by the agreement of line switching and packet switching services for both channels. Specifically, in a case where the B1 channel and the B2 channel of the interface are both utilized in the packet switching service, the line switching service will no longer be utilizable. This means that important voice communication or data communication cannot be utilized in the prior art.